If Docker Hub access is restricted via your Registry Access Management (RAM) configuration, you will not be able to pull images originating from Docker Hub even if the images are available in your registry mirror.

You will encounter the following error:
```console
Error response from daemon: Access to docker.io has been restricted by your administrators.
```

If you are unable to allow access to Docker Hub, you can manually pull from your registry mirror and optionally, retag the image. For example:
```console
docker pull <your-registry-mirror>[:<port>]/library/busybox
docker tag <your-registry-mirror>[:<port>]/library/busybox:latest busybox:latest
```

## How does it work?

The first time you request an image from your local registry mirror, it pulls
the image from the public Docker registry and stores it locally before handing
it back to you. On subsequent requests, the local registry mirror is able to
serve the image from its own storage.

### What if the content changes on the Hub?

When a pull is attempted with a tag, the Registry checks the remote to
ensure if it has the latest version of the requested content. Otherwise, it
fetches and caches the latest content.

### What about my disk?

In environments with high churn rates, stale data can build up in the cache.
When running as a pull through cache the Registry periodically removes old
content to save disk space. Subsequent requests for removed content causes a
remote fetch and local re-caching.

To ensure best performance and guarantee correctness the Registry cache should
be configured to use the `filesystem` driver for storage.

## Run a Registry as a pull-through cache

The easiest way to run a registry as a pull through cache is to run the official
[Registry](https://hub.docker.com/_/registry) image.
At least, you need to specify `proxy.remoteurl` within `/etc/docker/registry/config.yml`
as described in the following subsection.

Multiple registry caches can be deployed over the same back-end. A single
registry cache ensures that concurrent requests do not pull duplicate data,
but this property does not hold true for a registry cache cluster.

### Configure the cache

To configure a Registry to run as a pull through cache, the addition of a
`proxy` section is required to the config file.

To access private images on the Docker Hub, a username and password can
be supplied.

```yaml
proxy:
  remoteurl: https://registry-1.docker.io
  username: [username]
  password: [password]
```

> [!WARNING]
>
> If you specify a username and password, it's very important to understand that
> private resources that this user has access to Docker Hub is made available on
> your mirror. You must secure your mirror by implementing authentication if
> you expect these resources to stay private!

> [!WARNING]
>
> For the scheduler to clean up old entries, `delete` must be enabled in the
> registry configuration.

### Configure the Docker daemon

Either pass the `--registry-mirror` option when starting `dockerd` manually,
or edit [`/etc/docker/daemon.json`](/reference/cli/dockerd.md#daemon-configuration-file)
and add the `registry-mirrors` key and value, to make the change persistent.

```json
{
  "registry-mirrors": ["https://<my-docker-mirror-host>"]
}
```

Save the file and reload Docker for the change to take effect.

> [!NOTE]
>
> Some log messages that appear to be errors are actually informational
> messages.
>
> Check the `level` field to determine whether the message is warning you about
> an error or is giving you information. For example, this log message is
> informational:
>
> ```text
> time="2017-06-02T15:47:37Z" level=info msg="error statting local store, serving from upstream: unknown blob" go.version=go1.7.4
> ```
>
> It's telling you that the file doesn't exist yet in the local cache and is
> being pulled from upstream.